The viscosity of oils of lubricating viscosity is generally dependent upon temperature. As the temperature of the oil is increased, the viscosity usually decreases, and as the temperature is reduced, the viscosity usually increases.
The function of a viscosity improver (viscosity modifier, or viscosity index improver) is to reduce the extent of the decrease in viscosity as the temperature is raised or to reduce the extent of the increase in viscosity as the temperature is lowered, or both. Thus, a viscosity improver ameliorates the change of viscosity of an oil containing it with changes in temperature. The fluidity characteristics of the oil are improved since the oil maintains a more consistent viscosity over a wider range of temperatures.
It is desirable that the viscosity improver not adversely affect the low-temperature viscosity of the lubricant containing same. Frequently, while many viscosity improvers enhance the high temperature viscosity characteristics of lubricating oil, the low temperature properties of the treated lubricant become worse.
Additives that provide viscosity improving properties are known in the art. Such products are described in numerous publications including C. V. Smalheer and R. K. Smith “Lubricant Additives,” Lezius-Hiles Co. (1967).
International Patent Publication No. WO 02/10276, Feb. 7, 2002, discloses a mixture comprising A) a copolymer comprising about 70 to 79% by weight of units derived from ethylene, and having (a) Mw of 50,000 up to less than 130,000 and/or (f) SSI≦18; (b) density (D) of about 845 to about 895 kg/m3; (c) Mw/ Mn less than 3; (d) melting point (Tm) of about 15° C. to about 60° C.; and (e) degree of crystallinity≧1.5%, and B) an amorphous polymer having certain Mw and crystallinity properties. Also, additive concentrates and lubricating compositions comprising the components making up the mixture are disclosed.
It is also common that viscosity modifiers, which are typically polymeric materials are supplied in the form of a concentrate in oil, for convenience in use and handling. It is desirable for economic reasons that the concentration of the viscosity modifier in the concentrate be as high as possible, while retaining a reasonable viscosity so that the concentrate can be readily handled, e.g., poured or pumped. However, it has been observed that certain viscosity modifiers, in particular, certain hydrogenated styrene-diene block copolymers, e.g., styrene-isoprene block copolymers and star copolymers, cannot be used in concentrates at concentrations greater than about 6% by weight, since they lead to extremely high kinematic viscosities at 100° C., e.g., values in excess of 10,000 mm2/s (cSt), while ideally viscosities less than about 1000 mm2/s (cSt) are desirable.
Such hydrogenated diene/vinyl aromatic polymers are known, and concentrates of such polymers of improved viscosity are also known. U.S. Pat. No. 5,747,433, Luciani et al., May 5, 1998, discloses a composition of about 2 to about 20 percent of a hydrogenated diene/vinyl aromatic block copolymer and a selected non-ionic surface active agent, in a medium of oil of lubricating viscosity, which exhibits reduced viscosity compared with comparable compositions without the surface active agent.
U.S. Pat. No. 4,194,057, Brankling et al, Mar. 18, 1980, discloses a viscosity index improver additive composition containing a vinyl aromatic/conjugated diene polymer and an ethylene C3 to C18 alpha olefin copolymer.
U.S. Pat. No. 6,525,007, Okada et al., Feb. 25, 2003, discloses a viscosity modifier for lubricating oil, comprising an ethylene/propylene copolymer having 70-79% recurring units derived from ethylene, a weight average molecular weight of 80,000 to 250,000 and certain Mw/Mn and melting point characterizations. Other materials can be present, including as pour point depressants, copolymers of α-olefins and styrene.
European Patent Application EP 1 178 102, Feb. 6, 2002, discloses a lubricating oil composition of a lubricating base oil and a copolymer of ethylene and an α-olefin of 3 to 20 carbon atoms, having an ethylene content of 40-77% by weight and other characterizing parameters. Pour point depressants can also be present, such as copolymers of α olefins and styrene.
PCT Publication WO 96/17041, Jun. 6, 1996, discloses polymer blends containing olefin copolymers and star branched polymers, useful for improving the viscosity index of lubricating oils. The olefin copolymers can be ethylene-propylene copoivmer. The star polymers can be styrene/isoprene based polymers.
The present invention provides a blend of polymers such that high total levels of polymer in a concentrate can be employed, including relatively high levels of hydrogenated styrene-diene block copolymers, without leading to excessive concentrate viscosity. Moreover, in many instances the present invention provides unexpectedly good shear stability and low temperature performance to lubricant formulations.